Well completion techniques normally require perforation of the ground formation surrounding the borehole to facilitate the flow if interstitial fluid (including gases) into the hole so that the fluid can be gathered. In boreholes constructed with a casing such as steel, the casing must also be perforated. Perforating the casing and underground structures can be accomplished using high explosive charges. The explosion must be conducted in a controlled manner to produce the desired perforation without destruction or collapse of the well bore.
Hydrocarbon production wells are usually lined with steel casing. The cased well, often many thousands of feet in length, penetrates varying strata of underground geologic formations. Only a few of the strata may contain hydrocarbon fluids. Well completion techniques require the placement of explosive charges within a specified portion of the strata. The charge must perforate the casing wall and shatter the underground formation sufficiently to facilitate the flow of hydrocarbon fluid into the well as shown in FIG. 1. However, the explosive charge must not collapse the well or cause the well casing wall extending into a non-hydrocarbon containing strata to be breached. It will be appreciated by those skilled in the industry that undesired salt water is frequently contained in geologic strata adjacent to a hydrocarbon production zone, therefore requiring accuracy and precision in the penetration of the casing.
The explosive charges are conveyed to the intended region of the well, such as an underground strata containing hydrocarbon, by multi-component perforation gun system (“gun systems,” or “gun string”. The gun string is typically conveyed through the cased well bore by means of coiled tubing, wire line, or other devices, depending on the application and service company recommendations. Although the following description of the invention will be described in terms of existing oil and gas well production technology, it will be appreciated that the invention is not limited to those application.
Typically, the major component of the gun string is the “gun carrier” tube component (herein after called “gun”) that houses multiple shaped explosive charges contained in lightweight precut “loading tubes” within the gun. The loading tubes provide axial circumferential orientation of the charges within the gun (and hence within the well bore). The tubes allow the service company to preload charges in the correct geometric configuration, connect the detonation primer cord to the charges, and assemble other necessary hardware. The assembly is then inserted into the gun as shown in FIG. 2. Once the assembly is complete, other sealing connection parts are attached to the gun and the completed gun string is lowered into the well bore by the conveying method chosen.
The gun is lowered to the correct down-hole position within the production zone, and the chares are ignited producing an explosive high-energy jet of very short duration (see FIG. 3). This explosive jet perforates the gun and well casing while fracturing and penetrating the producing strata outside the casing. After detonation, the expended gun string hardware is extracted form the well or release remotely to fall to the bottom of the well. Oil or gas (hydrocarbon fluids) then enters the casing through the perforations. It will be appreciated that the size and configuration of the explosive charge, and thus the gun string hardware, may vary with the size and composition of the strata, as well as the thickness and interior diameter of the well casing.
Currently, cold-drawn or hot-drawn tubing is used for the gun carrier component and the explosive charges are contained in an inner, lightweight, precut loading tube. The gun is normally constructed from a high-strength alloy metal. The gun is produced by machining connection profiles on the interior circumference of each of the guns ends and “scallops,” or recesses, cut along the gun's outer surface to allow protruding extensions (“burrs”) created by the explosive discharge through the gun to remain near or below the overall diameter of the gun. This method reduces the chance of burrs inhibiting extraction or dropping the detonated gun. High strength materials are used to construct guns because they must withstand the high energy expended upon detonation. A gun must allow explosions to penetrate the gun body, but not allow the tubing to split or otherwise lose its original shape Extreme distortion of the gun may cause it to jam within the casing. Use of high strength alloys and relatively heavy tube wall thickness has been used to minimize this problem.
Guns are typically used only once. The gun, loading tube, and other associated hardware items are destroyed by the explosive charge. Although effective, guns are relatively expensive. Most of the expense involved in manufacturing guns is the cost of material. These expenses may account for as much as 60% or more of the total cost of the gun. The oil well service industry has continually sought a method or material to reduce the cost while also seeking to minimize the possibility of misdirected explosive discharges or jamming of the expended gun within the well.
Although the need to ensure gun integrity is paramount, efforts have made to use lower cost steel alloys through heat-treating, mechanical working, or increasing wall thickness in lower-strength but less expensive materials. Unfortunately, these efforts have seen only limited success. Currently, all manufacturers of guns are using some variation of high strength, heavy-wall metal tubes.